1. Field of the Invention
This invention relates generally to semiconductor lasers and, more particularly, to grating structures within semiconductor lasers.
2. Description of the Related Art
The use of compact photonic integrated circuits (PICs) has become widespread as the need for long-distance high-speed telecommunication systems has grown. Such long-distance systems, e.g. having optical fiber spans greater than 80 km, are limited in capacity by the optical channel spacing of carrier wavelengths and by the modulation format utilized. On-Off keying (OOK) modulation formats, for example, interact strongly with optical fiber nonlinearities to be useful at high data rates and close channel spacings. Accordingly, to maximize system throughput, phase modulation formats, such as differential quadrature phase shift keying (DQPSK) or differential phase shift keying (DPSK) for example, or polarization multiplexed versions of such formats, such as polarization multiplexed differential quadrature phase-shift keying (PM DQPSK) and polarization multiplexed differential phase-shift keying (PM DPSK), respectively, are often utilized with a channel spacing of 50 GHz, 25 GHz or less. However, to transmit and receive sufficiently error-free data under these conditions, a sufficiently narrow linewidth laser source is often used. For example, a laser source having a strong output at the fundamental mode, the lowest order mode resonant with the overall grating structure for example, while concurrently suppressing modes other than the fundamental mode is highly desirable. Laser sources utilized as local oscillators in coherent systems also may benefit from having a narrow linewidth, as compared to other conventional laser sources for example. In coherent systems the optical signal of the laser source, e.g. the local oscillator, is compared with an incoming signal to demodulate the data carried by the incoming stream.
To date, four basic types of lasers suitable for such high-speed applications utilizing phase modulation formats include: external feedback lasers; distributed feedback (DFB) lasers; distributed Bragg reflector (DBR) lasers; and discrete-mode Fabry-Perot or photonic bandgap lasers. While all of these laser sources offer significant attributes, each has one or more deficiencies which make them non-ideal for large-scale photonic integration, such as part of PIC device for example. For example, external-feedback lasers, by design, are incapable of monolithic integration. DFB lasers must be excessively large to achieve narrow linewidth, and thus consume large amounts of power and require a correspondingly large area of the integrated circuit, area which could be used for other components or for additional devices as part of a fabricated wafer to increase overall yield. Additionally, such long DFB lasers result in an exponential increase in energy at the center of the laser cavity resulting in spatial hole burning and gain loss with respect to the fundamental mode of the laser if the grating coupling strength is not adjusted. DBR lasers are susceptible to mode-hopping, for example jumping from one longitudinal mode to another due to operating conditions such as temperature, and require complex tuning algorithms with 3 or more sections contacted to have stable operation over life. Photonic bandgap lasers fabricated to date require cleaves and/or suffer from unpredictable yield, and therefore are unsuitable for large-scale photonic integration.
In designing suitable lasers having adequate power for high-speed optical transmission utilizing more advanced phase-shift modulation formats, a length of the laser cavity and a coupling coefficient κ of the grating must be considered. For example, in a DFB laser, characteristics of the optical output significantly change with respect to κL, representing the product of the coupling coefficient κ and the length L of the laser. Thus, in order to maintain a desired ratio between the output power and peak power of the laser while increasing the length of the laser, the coupling coefficient κ must be reduced accordingly.
What is needed is a semiconductor laser which is compact, offers narrow linewidth, low power consumption, and high yield, the semiconductor laser being suited for high-speed transmission of data utilizing phase modulation formats for example. It is also desirable for such a semiconductor laser to have reduced complexity, having a single contacted section spanning the entire laser for example. Also, what is needed is a semiconductor laser structure which provides for a reduced κ to maintain a desired ratio between the output power and the peak power of the laser. Further, what is needed is a photonic integrated circuit incorporating a plurality of such semiconductor lasers on a single substrate.